1. Field of the Invention
The present invention relates to a focusing servo system and a focusing servo acquisition enable method and, more particularly, to a focusing servo system for an optical recording/reproducing apparatus, and a focusing servo acquisition enable method.
2. Description of the Related Art
In an optical recording apparatus for recording information on a disk-shaped recording medium, such as an optical disk or a magnetooptic disk, or an optical reproducing apparatus for reproducing information recorded on a recording medium, the light beam emitted by an optical head must be focused correctly on the recording surface of the recording disk. Therefore, the optical recording apparatus or the optical reproducing apparatus is provided with a focusing servo system which drives the objective lens of the optical head for movement toward and away from the recording surface of the recording disk. Since a range in which a focusing servo action is possible, i.e., a focus acquisition enable range, is comparatively narrow, the objective lens is moved into the focus acquisition enable range by a focus search operation at the start of recording/reproducing operation or after track access, and then a focusing servo loop is turned on to carry out the focusing servo action. When starting recording operation or reproducing operation, spindle servo action and tracking servo action are executed after a focus search action and a focusing servo action have been executed. After the starting procedure has been completed, the scanning operation of the light beam for recording or reproducing is possible.
During the focus search operation, for example, the objective lens is moved forcibly between a position farthest from the surface of the disk and a position nearest to the surface of the disk, in which a focus error signal E.sub.F represented by an S-shaped curve in FIG. 1(b) is obtained by processing the output of the four-division detector of the optical head for detecting reflected light. FIG. 1(a) shows the sum signal, i.e., a signal R.sub.F, of the four-division detector. The signal R.sub.F is compared with a given threshold Th to obtain a FOK signal as shown in FIG. 1(c), which indicates a focusing servo enable range. After the objective lens is moved into the focus acquisition enable range corresponding to a period H in which the FOK signal is HIGH by the focus search operation, the focusing servo system is actuated for correct focusing control; that is, focusing servo control is executed to adjust the objective lens to the trailing edge of a focus zero crossing detection signal FZC shown in FIG. 1(d) in the focus acquisition enable range.
The focus search operation will be described hereinafter with reference to a flow chart shown in FIG. 2 and waveform diagrams of FIGS. 3(a) to 3(c) showing waveforms in different modes of focus search operation.
Referring to FIG. 2, a focus search operation is started in step F10, the focusing servo system is initialized in a time .DELTA.t.sub.1 =200 msec, i.e., a period between times T.sub.1 and T.sub.2 in FIGS. 3(a) to 3(c), in step F11. In the time .DELTA.t.sub.1, the rotation of the spindle rises and the objective lens is set at an initial position. Then, an initial search-up operation is executed in a time .DELTA.t.sub.2 =500 msec, i.e., a period between times T.sub.2 and T.sub.3 in FIGS. 3(a) to 3(c), in step F12 to move the objective lens to a position nearest to the disk in order that a search operation is started from the position nearest to the disk.
After the initializing operations have been completed, actual search operation, namely, an operation for detecting a focus acquisition enable range, is executed while the objective lens is moved. First a down-search operation, i.e., an operation to move the objective lens away from the disk from the position nearest to the disk, is executed in step F13, in which a search driving voltage is applied to a focusing coil at time T.sub.3 (FIGS. 3(a) to 3(c)) for down-search. During the down-search operation, a FOK signal is detected in search of a focus point in step F14. For example, if the FOK signal goes HIGH and a focus acquisition enable range is detected at time T.sub.4 (FIG. 3(a)), namely, if the response to a query in step F14 is affirmative, a focusing servo loop for correctly focusing the objective lens is closed in step F16. If the focus acquisition enable range is detected successively (FIG. 3(a)), the focus search operation is terminated in step F19.
If the focus point could not detected by the down-search operation, the down-search operation is continued until the objective lens reaches the farthest position, and then an up-search operation, i.e., a search operation to move the objective lens from the farthest position toward the disk, is executed; that is, the down-search operation is continued for a predetermined time .DELTA.t.sub.d (FIG. 3(b)), and then the up-search operation is started (steps F15 and F18). As shown in FIG. 3(b), the up-search operation is performed for a time .DELTA.t.sub.u between times T.sub.5 and T.sub.6, in which the objective lens is moved toward the nearest position, and then the down-search operation is started again at time T.sub.6 in step F13 to detect the focus point. The flow chart of FIG. 2 shows a procedure by which the focus point cannot be detected by the up-search operation by way of example. FIG. 3(b) shows an operation by which focus acquisition is achieved at time T.sub.7 and the focus search operation is terminated (steps F14.fwdarw.F16.fwdarw.F17.fwdarw.F19).
In some cases, as shown in FIG. 3(c), the focusing servo system fails in achieving focus acquisition due to disturbance or the like even if the focus point is detected. FIG. 3(c) shows a case in which a focus acquisition enable operation is started at time T.sub.4, but the focusing servo system fails in focus acquisition and the FOK signal goes LOW. Upon the detection of the change of the FOK signal from HIGH to LOW, the focus search operation is started again at time T.sub.11 ; that is the procedure returns from step F17 to step F11. Then, the initializing operation is executed between times T.sub.11 and T.sub.12 in step F11, the initial up-search operation is executed between times T.sub.12 and T.sub.13 in step F12 and the focus acquisition enable operation is carried out in a down-search mode. For example, the focus-search operation is achieved successfully at time T.sub.14 and the focus search operation is terminated.
Incidentally, it is important to complete the the focus search operation quickly. The quick focus search operation reduces the time necessary for starting the optical disk reproducing system and enables the optical disk reproducing system to start reproducing operation quickly.
An optical disk reproducing system stores data read from a disk temporarily at a high rate in a buffer memory, reads the data at a comparatively low rate from the buffer memory and converts the data read from the buffer memory into audio signals in predetermined timing so that the reproduced audio signals will not be interrupted even if the pickup head is dislocated from a correct tracking position by vibrations or disturbances. Since the data is stored in the buffer memory, the data accumulated in the buffer memory is read continuously even if the pickup head is dislocated from the correct tracking position and is disabled from reading data from the disk for a certain period of time. Therefore, the reproducing operation can be properly continued if track access operation is carried out and the reproducing operation is restarted before the data accumulated in the buffer memory is exhausted.
If the optical disk reproducing apparatus takes a comparatively long time in completing the starting operation after track access has been achieved, the data accumulated in the buffer memory decreases accordingly. For example, if tracking error occurs repeatedly in a short time, the buffer memory will be exhausted and reproduced sound will be interrupted. Thus, the delay in the starting operation is a significant problem particularly in an automotive optical disk apparatus which is subjected to vibrations.
Accordingly, the reduction of time necessary for starting operation has been an urgent problem and there has been a strong demand for the enhancement of the speed of focus search operation, which is effective in quickly completing the starting operation.
However, the foregoing known focus search operation needs a time before starting the focus acquisition enable operation again after the focus acquisition enable operation has not been achieved as shown in FIG. 3(c). For example, suppose that the initializing period .DELTA.t.sub.1 =200 msec, the initial up-search period .DELTA.t.sub.2 =500 msec, the period between T.sub.4 to T.sub.11 necessary for detecting a failure in the focus acquisition enable operation is 50 msec, and the period from T.sub.13 to T.sub.14 when the focus acquisition enable range is detected is 450 msec, the focus acquisition enable operation will not be restarted for about 1200 msec after the initial focus acquisition enable operation has resulted in failure. Since such a long time elapses before the focus acquisition enable operation is restarted, the focus search operation will take a very long time and the reproduced sound will be interrupted if a failure in the focus acquisition enable operation occurs several times successively.
The focus search operation may be quickly completed is the search speed, i.e., the moving speed of the objective lens, is increased. However, if the search speed is excessively high, it is impossible to control the position of the objective lens according to the focus acquisition enable range (FOK signal). Therefore, the enhancement of the search speed is limited.